1. Field of the Invention
The present invention relates to a piezoelectric actuator, and to a timepiece and a portable device including the piezoelectric actuator.
2. Description of the Related Art
Since piezoelectric elements have excellent responsiveness and conversion efficiency from electrical energy to mechanical energy, various types of piezoelectric actuators utilizing the piezoelectric effect of piezoelectric elements have been developed in recent years. The piezoelectric actuators have been applied to the fields of piezoelectric buzzers, ink-jet heads of printers, and ultrasonic motors.
FIG. 61 is a plan view schematically showing an ultrasonic motor using a conventional piezoelectric actuator. As shown in the figure, the ultrasonic motor of this type is called a poking type in which a rotor surface is slightly inclined and brought into contact with a tip of a vibrating piece connected to a piezoelectric element. In such a construction, when the piezoelectric element is expanded and contracted by an alternating voltage from an oscillator, and the vibrating piece reciprocates in a longitudinal direction, a force component is generated in a circumferential direction of the rotor and the rotor is rotated.
In addition, a technique has been known in which two ultrasonic vibrators (piezoelectric elements) are included, the ultrasonic vibrators vibrate with their own electrical resonance frequencies, and a vibrating piece is displaced by the vibration (Japanese Unexamined Application Publication No. 10-25151).
However, while the displacement of the piezoelectric element depends on the applied voltage, it is very small, usually about sub-micron, and this also applies to a case where the piezoelectric element vibrates with the above-described resonance frequency. For this reason, the displacement is amplified by a certain amplification mechanism, and is transmitted to the rotor. When the amplification mechanism is used, however, energy is consumed to operate the amplification mechanism, efficiency is lowered, and the size of an apparatus increases. In addition, when the amplification mechanism is used, it may be difficult to stably transmit a driving force to the rotor.
In addition, since a small portable device, such as a wristwatch, is driven by a battery, it is necessary to lower the consumption of electrical energy and the drive voltage. Therefore, when a piezoelectric actuator is incorporated into such a portable device, it is particularly required that the energy efficiency be high and the drive voltage be low.
Incidentally, in a calendar display mechanism for displaying the date, the day, and so forth in a timepiece or the like, it is common for the rotational driving force from an electromagnetic stepping motor to be intermittently transmitted to a date indicator or the like via a watch-hand-driving wheel train so as to advance the date indicator or the like. On the other hand, since the wristwatch is carried by being strapped on a wrist, a reduction in thickness for convenience of carrying has long been demanded. In order to pursue the reduction in thickness, it is also necessary to reduce the thickness of the calendar display mechanism. However, since the stepping motor is constructed by incorporating parts, such as a coil and a rotor, thereinto in an out-of-plane direction, the reduction in thickness of the calendar display mechanism is limited. For this reason, there is a problem in that the conventional calendar mechanism using the stepping motor is not structurally suited for reducing the thickness.
In particular, in order to share a mechanical system (a so-called movement) between a timepiece with a calendar display mechanism and a timepiece without such a display mechanism, it is necessary to construct the calendar display mechanism on the side of a dial. However, it is difficult for an electromagnetic stepping motor to achieve a reduction in thickness to such an extent that the calendar mechanism can be constructed on the side of the dial. Therefore, it is necessary for a conventional timepiece to be manufactured by separately designing watch-hand-driving mechanical systems according to whether there is a display mechanism, and this becomes a problem when improving the productivity thereof.
The present invention is made in consideration of the foregoing circumstances, and an object is to provide a piezoelectric actuator that facilitates a reduction in size by simplifying conductive construction, and to provide a timepiece and a portable device including the same. In addition, it is an object to provide a piezoelectric actuator that is able to efficiently transmit vibrations of a piezoelectric element, that is suited for a reduction in size and thickness, and that is able to stably transmit a driving force, and to provide a timepiece and a portable device including the same.